The invention described herein was made in the course of, or under, a contract RP8012-11 with the Electric Power Research Institute.
1. Field of the Invention
This invention pertains generally to monitoring and controlling the freezing of foods, and more particularly to a non-invasive apparatus and method for using nuclear magnetic resonance imaging to determine the ice interface in the object which, together with the air temperature and air velocity, are used to measure enthalpy in food freezing operations. Alternatively, enthalpy can be measured from the ratio of liquid to solid portions of the object determined from nuclear magnetic resonance imaging or, for objects of substantially symmetrical geometric shape, from the liquid/solid ratio obtained by integrating the amplitude of nuclear magnetic resonance signals.
2. Description of the Background Art
Freezing is one of the major means of food preservation in industrialized countries of the world. The types of products which are frozen range from agricultural to animal food stuffs, and include fruits, vegetables, animal meats, fish, and dairy products. In addition to raw commodities, many formulated or "value-added" products are frozen. These include popular items such as pizza, TV dinners, ice cream, and fish sticks.
Preservation of food requires that it be maintained in a safe and nutritious state, and still taste good to the consumer. One of the primary goals of freezing is to limit the growth of microorganisms during prolonged storage. While freezing does not necessarily kill bacteria and mold, low temperatures associated with freezing limit metabolic processes necessary for them to duplicate. Excessive microbial loads may be reduced prior to freezing through blanching or other thermal treatments.
Temperature reduction during freezing also decreases the rate of chemical reactions that continue during storage. This impacts nutritional and sensory attributes, as many of the reactions may be degradative. For example, vitamins may break down, pleasing color attributes may change, or off-flavors may develop all due to chemical processes. Many frozen and thawed foods, however, are often indistinguishable from the fresh commodity. Overall, lowering the temperature of a food while converting water to ice helps limit the growth of microorganisms, slows deleterious chemical reactions and, in some cases, imparts desirable sensory qualities to the food.
Many types of freezers are available for food freezing. The type of freezer used varies with the product being frozen based on such factors as the final temperature to be reached, the size of the product, the thermal properties of the product, whether the pieces are to be frozen individually or as a group, and the amount of space available. Typically, freezers can be categorized as either batch or continuous types. In batch processes, the product is placed into the freezer and left to sit for a given time. In continuous processes, the product is conveyed through the length of the freezer on carts or conveyor belts. Combinations of batch and continuous modes are also possible.
A critical factor in all freezing operations is the "residence time" of the product; that is, the time in which the product remains in the freezer. For continuous freezers, this depends on the length of the freezer and the speed at which product is moved through the freezer. For batch freezers, this entails the amount of time the product is left in the freezer before removal. Residence time is a critical factor in food freezing, since a too short residence time will mean that the product has not reached its desired temperature. Conversely, a prolonged residence time means that product throughput could actually be increased.
The primary means of lowering temperature in freezers is through the compressor-based refrigeration cycle. Liquid refrigerant at low temperature and pressure is circulated through a heat exchanger (evaporator) in the freezer compartment. As heat is removed from the compartment, the refrigerant becomes a low temperature gas. A compressor converts this to a gas at high temperature and pressure. The gas condenses as it passes through a second heat exchanger (condenser) where the heat is exhausted to the surroundings. The cycle is completed as it passes through an expansion valve.
No hard data exist on the energy requirements for industrial freezing operations. However, an order of magnitude estimate can be made as follows. Assume a food product is cooled and frozen from 30.degree. C. to -15.degree. C. From ASHRAE tables, an average specific heat between 32.degree. C. and 4.degree. C. is about C.sub.p =3.6 kJ/kg .degree.C. A typical latent heat change between 0.degree. and -15.degree. C. would be 305 kJ/kg. Thus, the heat removed to cool and freeze 1 kg is EQU 25.times.3.6 kj/kg .degree.C.+15.times.305 kJ/kg=395 kJ/kg
or 147 kJ/lb. The true refrigeration load in a freezer is larger, as heat may be gained from the surroundings through insulation leaks and air changes, or through waste heat from lights, motors, or even people. As a conservative estimate, we assume these add 10% to the refrigeration load, or EQU 395 kJ/kg+0.10.times.395 kJ/kg=435 kJ/kg
For most industrial freezing operations, energy is expended in the compressor to compress refrigerant vapor. In essence, electrical energy is delivered to the compressor in order to continue the refrigeration cycle. The ratio of heat energy absorbed in the evaporators to the energy input to the compressor is given by the coefficent of performance (COP): ##EQU1## Typical COPs for industrial refrigerants range from 4.59 for R-22 to 4.77 for ammonia, given an evaporating temperature of -15.degree. C. and a condensing temperature of 30.degree. C. These values account for the inefficiency of compression, such as, losses in the compressor clearance volume. Using an average of 4.65, the minimal electric energy input for the situation described above is: EQU 435 kJ/kg.div.4.65=94 kJ/kg
Next, if one assumes the compressor motor is 80% efficient, the electrical consumption becomes EQU 94 kJ/kg.div.0.80=117 kJ/kg
Typically, there is about a 30% additional energy cost from such things as circulating air through blast freezers or moving product on conveyors. This increases electrical consumption to EQU 117 kJ/kg+0.30.times.117 kJ/kg=152 kJ/kg
or 57 kJ/lb of frozen food. In 1990, about 28 billion pounds of food were frozen. Thus, we might estimate the electrical consumption for food freezing to be on the order of EQU 57 kJ/lb.times.28.times.10.sup.9 =1.6.times.10.sup.12 kJ
or about 4.5.times.10.sup.8 kW.multidot.hr per year.
The goal in freezing is to remove enough heat from the product to bring it to its desired enthalpy value. The enthalpy of the product upon removal from the freezer should be the same as the value it has during frozen storage. If residence time is too short and exit enthalpy is too high, further heat will be removed during storage. This is undesirable as facilities designed for frozen storage are not as efficient at removing heat as freezers. If freezer residence time is too long, excess electrical power will be consumed without further reduction in product enthalpy. As an example, if a product is sufficiently frozen in 15 minutes but remains in a freezer for 18 minutes, the operation consumes 20% more energy than necessary. If energy conservation of just 1% could be realized, this could mean savings on the order of 4.5 million kW.multidot.hr per year.
Freezing rates or freezing times can be defined in a variety of ways, including:
(a) the change in temperature with time at some point (dT/dt); PA1 (b) the velocity at which the ice interface advances (dx/dt); PA1 (c) the total time required to traverse a given temperature interval (t.sub.f); PA1 (d) the rate of enthalpy change (dH/dt) or time required for a given enthalpy change.
Traditionally, the primary means of measuring or predicting food freezing rates have been through thermometric measuring during freezing, or mathematical modelling. In the former, the time required for a given temperature change to occur is assessed by placing thermocouples within a sample during freezing. This provides a recorded history of temperature profiles within the sample. Freezing rate has been defined in several ways, such as by dividing the surface to center distance by the time required to reach 0.degree. C. and the thermal center to reach 5.degree. C. below the freezing temperature. More common, however, is the concept of "freezing time" such as the time required for the slowest cooling point to decrease from 0.degree. C. to -5.degree. C. Use of thermocouples, however, is undesirable since they may provide additional heat conduction paths to the same or alter air flow patterns. Additionally, such an approach does not measure the variable of most interest, namely the change in enthalpy with time.
With regard to mathematical modelling, Planck's equation, the simplest of available freezing models, describes this time t.sub.f as: ##EQU2## where r is the density of unfrozen material, .DELTA.H.sub.L is the latent heat of water in the product, T.sub.f is the freezing temperature, T.sub..infin. is the final temperature, h is the surface heat transfer coefficient, k is the thermal conductivity of ice, a is a characteristic dimension, and P and R are shape factors. Planck's model is only a rough approximation to the freezing time, as it assumes all the latent heat is removed at one temperature, and that thermal conductivity is constant throughout the process. While Planck's model is a useful equation because it qualitatively describes the factors important to freezing rates, it is untenable to obtain an exact solution for heat conduction in a system undergoing a gradual phase change, and in which the pertinent physical properties vary with temperature.
However, when considering energy costs, defining freezing rates and freezing times in terms of enthalpy changes is more useful. At constant pressure, the enthalpy difference .DELTA.H is just the heat gained or lost in the process. For example, if a product is cooled and frozen from an initial temperature T.sub.R to a steady-state at temperature T.sub..infin., the enthalpy difference is the amount of heat removed by the freezer to bring the product to its steady-state. The time required for this to happen is subject to the tolerance specified for reaching the final state. Cooling and freezing represent a transient process approaching a steady-state. In practice, one could define the freezing time as that required for some percentage, say 95%, of the total change to be made.
Significantly, there are no in-line sensors available for assessing freezing rates or when freezing is complete. Measurements using conventional temperature probes within the product are necessarily invasive and are difficult to monitor on a routine basis. Conventional calorimeters, on the other hand, typically make steady-state measurements of products inside the calorimeter itself. As such, they give no information on the required freezing time. Therefore, there is a need for a method and apparatus for in-line, non-invasive assessment of the freezing process. The present invention satisfies this need, as well as others, and overcomes the deficiencies in prior devices.